The fields of manufacturing conventional printed wiring boards and precision working of metals widely employ photosensitive elements comprising a layer made of a photosensitive resin composition (“photosensitive layer”), a support film and a protective film, as resist materials for etching, plating and the like.
A printed wiring board may be manufactured, for example, in the following manner. First, the protective film of a photosensitive element is released from the photosensitive layer and the photosensitive layer is then laminated on the conductive film of a circuit-forming board. After subsequent pattern exposure of the photosensitive layer, the unexposed sections are removed with a developing solution to form a resist pattern. The conductive film is patterned based on this resist pattern to form the printed wiring board.
The developing solution used for removal of the unexposed sections is usually an alkali developing solution such as a sodium carbonate solution. For most purposes, the developing solution may be any one capable of dissolving the photosensitive layer to some extent, as the photosensitive layer becomes dissolved in the developing solution or dispersed in the developing solution during development.
With increasingly higher densities of printed wiring boards in recent years, the contact areas between circuit-forming boards and the photosensitive layers that are used as resist materials have been decreasing in size. The photosensitive layer must therefore have excellent mechanical strength, chemical resistance and flexibility in the etching or plating step, while also having excellent adhesiveness for circuit-forming boards and high resolution for pattern formation. In recent years, materials that can form resist patterns with line widths and space widths that are both 10 μm or smaller are in demand, particularly for use in package boards, and it is becoming necessary for resist pattern shapes to be nearly rectangular in order to increase the interconnect insulating reliability.
When a photosensitive element is used to form a resist, usually the photosensitive layer is laminated on the board and exposed without peeling off the support film. An optically transparent material may be employed as the support film to facilitate exposure treatment. The support film must also be as thin as possible in order to obtain high resolution for pattern formation. On the other hand, a certain minimum level of thickness (generally 10 μm-30 μm) is required for the support film in order to allow coating of the photosensitive resin composition on the support film to a uniform thickness with satisfactory yield. For increased productivity of the support film, i.e. to improve the take-up properties of the support film, inorganic or organic fine particles are usually added to the support film. Conventional support films therefore have increased haze, and the fine particles in the support film tend to cause light scattering under light exposure, so that the requirement for high resolution of the photosensitive film cannot be met.
Methods for achieving high resolution include a method wherein a support film provided on a photosensitive element is peeled off before exposure so that exposure is accomplished without the support film. In this case, a phototool is often bonded directly onto the photosensitive layer. However, since the photosensitive layer usually has some level of pressure-sensitive adhesive property, it is difficult to remove the bonded phototool when exposure is carried out after directly bonding a phototool onto the photosensitive layer. In addition, the photosensitivity tends to be reduced by contamination of the phototool by the photosensitive layer, or exposure of the photosensitive layer to oxygen in the air when the support film is removed.
Various means have been proposed as improvements in this regard. For example, Patent Literatures 1 and 2 disclose methods in which two or more photosensitive layers are formed, the layer directly bonded to the phototool being one without an adhesive property. Also, Patent Literatures 3 to 8 propose methods in which an interlayer is provided between the support film and photosensitive layer. In Patent Literatures 9 and 10 there are proposed methods in which inorganic or organic fine particles with a mean particle size of about 0.01 to 5 μm are added to the outer surface on one side of the support film to lower the haze, thus allowing high resolution even with exposure through a support film.